The present invention relates to a method for preventing color in hydrogenated diene polymers. More particularly, the invention relates to a method for preventing color in block copolymers of conjugated dienes and vinyl aromatic hydrocarbons that have been hydrogenated using titanium-based catalysts.
Titanium catalysts can now be used for hydrogenating diene polymers, especially block copolymers of conjugated dienes and vinyl aromatic hydrocarbons which require selective hydrogenation. These catalysts have advantages over the nickel catalysts used in the past in that less catalyst can be used to achieve the same amount of hydrogenation. This makes the removal of the catalyst easier and, in some cases, the catalyst residue may remain in the polymer because the amount is so low.
For instance, U.S. Pat. No. 5,039,755 describes a process for the hydrogenation of conjugated diolefin polymers which involves termination of the diolefin polymer with hydrogen. Then the polymer is hydrogenated in the presence of at least one bis(cyclopentadienyl)titanium compound of the formula: ##STR1## wherein R.sub.1 and R.sub.2 are the same or different and are selected from the group consisting of halogen groups, C.sub.1 -C.sub.8 alkyl and alkoxy groups, C.sub.6 -C.sub.8 aryloxy groups, aralkyl, cycloalkyl groups, silyl groups and carbonyl groups. The hydrogenation step is carried out in the absence of hydrocarbon lithium and alkoxy lithium compounds. Other titanium catalyst systems which can be used for the hydrogenation of such polymers include those disclosed in U.S. Pat. Nos. 4,501,857, 4,673,714 and 4,980,421.
In some cases, especially at high catalyst levels, a yellow color is imparted to the polymer. Conventional methods of removing this color such as full-scale acid extraction are not preferred because this deashing step is costly. Therefore, it is an object of the present invention to provide a method for hydrogenating diene polymers with a titanium hydrogenation catalyst and to produce a hydrogenated polymer that does not have a yellow color.